Chlorine dioxide and sodium hydroxide are widely used in the bleach plant of pulp mills for brightening and purifying pulp. Chlorine dioxide is produced on-site at the mill by reduction of sodium chlorate in an acid aqueous reaction medium, in accordance with the equation: EQU ClO.sub.3.sup.- +Cl.sup.- +2H.sup.+ .div.ClO.sub.2.sup.-+1/2Cl.sub.2 +H.sub.2 O
Sodium hydroxide usually also is formed on-site at the mill by electrolysis of aqueous sodium chloride solution in a divided cell, in accordance with the equation: EQU NaCl+H.sub.2 O.fwdarw.NaOH+1/2Cl.sub.2 +1/2H.sub.2
Sodium hydroxide is formed at the cathode and chlorine at the anode. The chlorine co-produced often has little value to the pulp mill.
One class of chlorine dioxide-generating process is one involving reaction of sodium chlorate with hydrochloric acid, in accordance with the equation: EQU NaClO.sub.3 +2HCl.fwdarw.ClO.sub.2 +1/2Cl.sub.2 +NaCl+H.sub.2 O
One example of such process is the so-called "R5" process, as described in Canadian Pat. No. 956,784 of the assignee hereof, the disclosure of which is incorporated herein by reference, wherein the reaction is effected in a boiling reaction medium having a total acid normality of about 0.05 to about 0.3 normal to which a subatmospheric pressure is applied. The resulting chlorine dioxide and chloride are removed from the reaction zone in admixture with steam. The process may be effected with precipitation of by-product sodium chloride or with removal of an aqueous effluent containing by-product sodium chloride.
Another example of such a chlorine dioxide-generating process is the electrolytic process described in U.S. patent application Ser. No. 156,965 filed Feb. 18, 1988, assigned to the assignee hereof and the disclosure of which is incorporated herein by reference. As described therein, externally-fed chlorate ions are reduced with hydrogen ions and chloride ions in the cathode compartment of an electrolytic cell having a three-dimensional high surface-area cathode separated from an anode compartment by a cation-exchange membrane. An electric current applied to the cell reduces co-produced chlorine in the cathode compartment to chloride ions while electrolytically-formed hydrogen ions, generally providing about one-half the acid requirement, are transferred across the cation-exchange membrane from the anode compartment to the cathode compartment. The process produces an aqueous sodium chloride by-product stream from the cathode compartment.